The present invention relates to silver halide imaging, and more particularly, to means and methods of silver halide imaging in diffusion transfer systems.
There are many imaging systems which employ silver halide emulsions as their photosensitive component. The emulsion is usually a suspension of microscopic silver halide grains in a continuous phase, such as gelatin. When the silver halide emulsion is exposed to actinic radiation, a chemical change occurs in the exposed silver halide grains that facilitates their reduction to metallic silver by a developer. ("Actinic radiation" as used in this specification and the appended claims means the entire spectrum of electromagnetic radiation.) This change in the exposed silver halide grains is known as a latent image. It is generally believed that the latent image consists of colloidal particles of silver, each containing several silver atoms, which are located primarily at the surface of exposed silver halide grains. (Colloidal particles are generally defined as having at least one dimension between about 1 nanometer and 1 micron.)
The number of changed, i.e. developable, grains in any small area of the emulsion depends on the intensity of the light exposure of that area. When a developer is applied, the chemical reduction of the exposed grains occurs. Therefore, after development, there is an image which is a silver deposit in the emulsion, and whose density varies across the surface of the emulsion in a pattern that corresponds to the image that was exposed on it.
The next step commonly employed in processing silver halide emulsion-type film is fixing, which removes the remaining unreduced grains to prevent their later darkening. The image that results from this process is negative, and a positive image is usually produced from it through contact printing or projection.
One alternative process for producing positive images is referred to as diffusion transfer. In this process a negative silver halide film is exposed to actinic radiation as described above. Typically, development is completed in an aqueous monobath which contains a developer, a strong base to aid in development, a silver transport agent, and a solvent. The transport agent and solvent dissolve the grains of silver halide in unexposed areas, some of the grains in lightly exposed areas, and almost no grains in highly exposed areas. The grains that are not dissolved are the ones that have been or are being reduced to metallic silver by the developer. The dissolved silver halide diffuses from the emulsion to a receiving or nucleating sheet superimposed on the transfer sheet. Alternatively, the nucleating area may be a separate layer on the same substrate as the silver halide emulsion.
The nucleating sheet or layer contains colloidal silver, which catalyzes the reduction of the dissolved silver halide to metallic silver. As a result, silver deposits form across the nucleating area in inverse proportion to the intensity of exposure of the corresponding area on the silver halide emulsion. Thus, a positive image is formed in the nucleating layer.
Attempts to include the developer, base and silver transport agent in a single, self-contained diffusion transfer sheet that can be activated by application of a solvent alone have met with a number of problems. Some of these three materials are incompatible with one another. For example, the base must be kept separate from the developer. If it is not kept separate, the developer will readily oxidize, thus substantially reducing the shelf life of the diffusion transfer sheet. Also, the silver transport agent must be separate from the silver halide emulsion in order to prevent premature silver dissolution.
The obvious expedient of coating these materials onto a single substrate as separate layers has not been shown to be entirely technically and commercially unsatisfactory. Some mixing inevitably occurs during coating, and absorbed atmospheric water causes the water soluble components to migrate. Therefore, no answer has apparently been found in the past to the problems of making a self-contained diffusion transfer sheet. This problem has limited the scope of diffusion transfer imaging's usefulness, since in some applications the complexity of a two-sheet system or applying the developing reagents from an external source presents a major difficulty. There has been a long felt need in the photographic and photocopy industries for products and processes which avoid or minimize the problems described above.
Another problem with prior art diffusion transfer systems involves the emulsion composition. The silver halide emulsion is commonly formed with gelatin, a highly ionic material, as the continuous phase. The use of gelatin limits the types of solvents that are satisfactory, because most non-aqueous solvents are insufficiently polar to overcome the attraction of the ionic silver grains to gelatin. Therefore, when gelatin is used with a non-aqueous solvent, little silver transport occurs.
U.S. Pat. No. 3,348,946 to Jones is an example of method and means for diffusion transfer imaging. A developer is incorporated in a silver halide emulsion on a substrate. A developing solution containing a polyhydric alcohol, a base and a silver halide stabilizing agent is applied to the emulsion sheet after exposure. Normally the developing solution is incorporated into a separate, image receiving sheet, which is superimposed on the emulsion sheet during development, and in which the positive image is ultimately produced.
Two other patents in the area of diffusion transfer imaging systems are British Pat. Nos. 1,182,302 and 1,182,306 to Land et al. These patents disclose multi-layer diffusion transfer sheets which include a substrate, a light-sensitive layer and a translucent layer. The sheets disclosed can also contain a silver receptive stratum and processing reagents. The translucent layer and light-sensitive layer can be combined as an alternative embodiment.
In the disclosure of these patents, an aqueous medium is used to dissolve and/or apply the processing reagents (developer, base, etc.). The light-sensitive layer is taught to comprise a silver halide emulsion with gelatin as its continuous phase.
The translucent layer blocks a high enough percentage of incident light that it prevents premature exposure of the silver halide emulsion, and also provides a background for viewing the positive image that is ultimately formed in the silver-receptive stratum. However, it permits enough light to pass so that exposure of the silver halide emulsion is not appreciably retarded when an exposure light source is used.